Institute of Electronic and Electrical Engineers (IEEE) standard 802.11ac provides an arrangement for channel estimation of uplink and downlink channels between an access point (AP) and multiple wireless stations (STAs). The channel estimation involves a process called sounding. To perform downlink sounding, an AP transmits a Null Data Packet-Announcement (NDP-A). The NDP-A contains the addresses of particular STAs to which the AP wishes to transmit data. The NDP-A is followed by a Null Data Packet (NDP). The NDP represents pilot energy; that is, a pre-defined pattern. Each STA addressed by the NDP-A measures the downlink wireless channel from the AP to itself. The channel measurement is possible because the given STA knows the pre-defined pattern and can estimate the effects of the channel on that pattern (for example, amplitude attenuation, carrier phase rotation, and time delay). Uplink sounding can be performed by instructing one or more STAs to send NDPs, which the AP then observes.
IEEE 802.11n specifies a high throughput (HT) physical layer and medium access control layer. IEEE 802.11ac specifies a very high throughput (VHT) physical layer and medium access control layer. More information can be found in IEEE P802.11 Wireless LANS, Specification Framework for TGax, Nov. 25, 2015, doc.: IEEE 802.11-15/0132r10.
For downlink channel measurements, various STAs feedback their channel measurements to an AP and an AP with multiple antennas can transmit multiple streams to the multiple STAs (each with multiple antennas) based on the received channel information.
Wireless Local Area Networks (WLANs) supporting uplink and downlink transmission between multiple STAs and APs rely on accurate channel information. An AP may have many STAs desiring service, with different radio channels to each STA due to unique scattering geometries between the AP and each STA. A conventional AP can allocate bandwidth of an uplink transmission to a STA that has a weak uplink channel to the AP. Other STAs will not be scheduled to transmit, and the overall system throughput is low.